Nearly all motor vehicles produced within the past few years have been equipped with seat belt retractors of the emergency locking type ("ELR"), i.e., of the type that allows the belt to be withdrawn at all times other than when an emergency locking mechanism responds to rapid acceleration (most commonly negative acceleration) of the vehicle or rapid rotational acceleration of the belt reel. ELR's have the advantage of allowing the occupant to lean forward.
It is known that even though the locking mechanism of an ELR, when activated, stops the belt reel from rotating in the unwinding direction, a small length of belt may be pulled from the reel due to tightening of the loops of the belt that remains wound on the reel. It has, accordingly, been proposed to provide a belt clamp to keep the belt from being unwound by tightening on the retractor reel. Examples of belt clamps for restricting pull-out from the reel of the segment of the belt that restrains the occupant are found in U.S. Pat. Nos. 4,544,112 (Ziv, 1985), 4,323,204 (Takada, 1982), 4,243,185 (Montonami et al., 1981) and 4,687,253 (Ernst et al., 1987).
Japanese Published Application No. 55-12923 (1980) describes and shows an ELR equipped with a belt clamp mechanism that is linked to and operated by an actuating plate that also engages a reel-locking bar with a main locking ratchet wheel affixed to the belt reel. A pendulum type inertia sensor moves the slidably mounted actuator plate upwardly when activated. A tooth on the plate is thereupon engaged by a secondary ratchet wheel that rotates with the reel. Upon a small amount of rotation of the reel, the actuating plate pivots the reel-locking bar into engagement with the reel and also pushes up on a link, which is coupled to a lever having a clamping jaw on its free end. The lever, which is normally biassed by a spring to an inactive position, is pivoted up to clamp the belt against a part of the frame and arrest its movement. The design has two main shortcomings. First, the pendulum inertia sensor is called up to act against the mass of the actuator plate, the link and the clamp lever, the force of the lever-biassing spring, and the friction of these several components and the sliding and pivoting mounts. This requires a comparatively large, heavy pendulum, and also introduces a high degree of imprecision in the sensitivity of the inertia sensor, particularly due to large friction effects which are somewhat unpredictable. Second, the design is limited to response to vehicle acceleration and is not suited to the presently preferred designs of ELR's which incorporate mechanisms that respond to both vehicle acceleration and belt reel acceleration. In addition the design of the belt clamp of Japanese Published Application No. 55-12923 is subject to diminishing effectiveness as the belt wears, because it relies on friction of the belt against a plain frame wall to restrain the belt.